Fuel cell power plants are well-known and are commonly used to produce electrical energy from reducing and oxidizing fluids to power electrical apparatus such as apparatus on-board space vehicles. In such power plants, a plurality of planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids. Each individual cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reactant or reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. In a cell utilizing a proton exchange membrane ("PEM") as the electrolyte, the hydrogen electrochemically reacts at a surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
The anode and cathode electrodes of such fuel cells are separated by different types of electrolytes depending on operating requirements and limitations of the working environment of the fuel cell. One such electrolyte is the aforesaid proton exchange membrane ("PEM") electrolyte, which consists of a solid polymer well-known in the art. Other common electrolytes used in fuel cells include phosphoric acid or potassium hydroxide held within a porous, non-conductive matrix between the anode and cathode electrodes. It has been found that PEM cells have substantial advantages over cells with liquid acid or alkaline electrolytes in satisfying specific operating parameters because the membrane of the PEM provides a barrier between the reducing fluid and oxidant that is more tolerant to pressure differentials than a liquid electrolyte held by capillary forces within a porous matrix. Additionally, the PEM electrolyte is fixed, and cannot be leached from the cell, and the membrane has a relatively stable capacity for water retention. As is well-known however, PEM cells have significant limitations especially related to liquid water transport to, through and away from the PEM, and related to simultaneous transport of gaseous reducing fluids and process oxidant fluids to and from the electrodes adjacent opposed surfaces of the PEM. The prior art includes many efforts to minimize the effect of those limitations.
In operation of PEM fuel cells, the membrane is saturated with water, and the anode electrode adjacent the membrane must remain wet. As hydrogen ions produced at the anode electrode transfer through the electrolyte, they drag water molecules in the form of hydronium ions with them from the anode to the cathode. Water also transfers back to the anode from the cathode by osmosis. Product water formed at the cathode electrode is removed from the cell by evaporation or entrainment into a circulating gaseous stream of either the process oxidant or reducing fluid. It is critical that a proper water balance be maintained between a rate at which water is produced at the cathode electrode and rates at which water is supplied to and removed from both electrodes. An operational limit on performance of a fuel cell is defined by an ability of the cell to maintain the water balance as electrical current drawn from the cell into the external load circuit varies and as an operating environment of the cell varies. For PEM fuel cells, if insufficient water is returned to the anode electrode, adjacent portions of the PEM electrolyte dry out thereby decreasing the rate at which hydrogen ions may be transferred through the PEM and also resulting in cross-over of the reducing fluid leading to local over heating. Similarly, if insufficient water is removed from the cathode, the cathode electrode may become flooded effectively limiting oxidant supply to the cathode and hence decreasing current flow. Additionally, if too much water is removed from the cathode by the gaseous stream of oxidant, the cathode may dry out limiting ability of hydrogen ions to pass through the PEM, thus decreasing cell performance, and limiting a useful life of the PEM.
In fuel cell power plants, preventing localized dry-out of the PEM is additionally complicated because the process oxidant is typically air, and therefore subject to varying relative humidity depending upon environmental conditions in which the plant is operated. Process oxidant air enters an operating fuel cell of the plant through a cathode inlet and then flows through a cathode flow field adjacent the cathode electrode, and out of the fuel cell through a cathode outlet. As the fuel cell operates, heat is generated at the cathode, and therefore the temperature of a process oxidant stream immediately rises as it enters the cathode inlet. That results in a drop of the relative humidity of the oxidant stream, which facilitates movement of water out of the PEM and into the oxidant stream adjacent and downstream of the cathode inlet.
It is well-known to use a cooling component adjacent the process oxidant stream for cooling the stream and thereby raising the relative humidity of the stream in order to minimize water movement out of the PEM and into the process oxidant stream. For example, U.S. Pat. No. 5,547,776 issued on Aug. 20, 1996 to Fletcher et al. shows use of a series of inactive humidification cells that utilize a water transport membrane to humidify fuel and oxidant streams prior to entry into the active fuel cells wherein a sealed coolant plate is positioned adjacent an oxidant flow field to direct an isolated coolant stream to cool the oxidant as it passes through the cell, and to remove heat from the cell. That approach provides a long, complicated and serpentine flow path for the oxidant stream through the humidification cells and cathode flow field which may be effective for a fuel cell power plant that pressurizes the process oxidant stream, but is impractical for a fuel cell power plant operated at about ambient pressure.
Further attempts to maintain an electrolyte saturated with water while enhancing efficient movement of fluids to, through and away from the membrane have included adding porous water transport plates adjacent porous support layers within anode and cathode flow fields to facilitate liquid water transport and cooling throughout the cell; integrating a condensing loop external to the cell to condense moisture within an exiting oxidant stream such as by a heat exchange relationship with ambient air and then returning the condensed moisture to the porous support layers adjacent the anode electrode; rendering a portion of a non-PEM, phosphoric acid electrolyte electrochemically inactive in a phosphoric acid cell and thereby forming a condensation zone adjacent an oxidant gas outlet which zone operates at a cooler temperature than the active portions of the electrolyte to thereby limit electrolyte loss (as shown in U.S. Pat. No. 4,345,008 to Breault and assigned to the assignee of the present invention); and generating a pressure differential on the anode side of the cell wherein the reducing fluid or fuel is maintained at a slightly higher pressure than coolant water and anode supply water passing through porous support layers adjacent reducing gas distribution channels so that the pressure differential assists water transport through porous support layers and the PEM.
While such improvements have enhanced fuel cell efficiencies, PEM fuel cells still suffer operational limits such as when the process oxidant stream enters the cell at a low relative humidity so that water may be evaporated from of the PEM adjacent the cathode inlet and into the oxidant stream at a rate faster than water can be replaced into the PEM by osmosis from product water or by water from the anode side. Such loss of water by the PEM leads to decreased cell performance, shorter cell life, and possible reactant gas cross over. Accordingly there is a need for a fuel cell power plant that maintains a high relative humidity throughout the cell thus resulting in a water saturated PEM throughout anticipated operating environments of the plant.